A stunning unfolding of international crises, from Iraq to Ukraine to Syria to Gaza, has prompted some less-than-edifying Washington debate: It’s all President Obama’s fault. No, it’s not his fault at all.

It would be a pity if partisan fervor kept us from learning from recent events, because in fact the available lessons are stark: We have witnessed as close to a laboratory experiment on the effects of U.S. disengagement as the real world is ever likely to provide.

Obama openly and deliberately adopted a strategy, not of isolationism, but of gradual withdrawal, especially from Europe and the Middle East. He argued that America should concentrate on “nation-building here at home.” He espoused a pivot to Asia, on the grounds that the Pacific region was the world’s most dynamic and deserving of U.S. military and diplomatic attention. (“Here, we see the future,” Obama told Australia’s parliament.)

Many policies followed:

●All U.S. troops were withdrawn from Iraq. Whether this was at the insistence of Prime Minister Nouri al-Maliki, as Obama’s defenders argue, or because Obama offered so few troops, and so half-heartedly, that Maliki was bound to reject the offer matters less than this: Obama was content with the zero option and, as he made clear at the time, sanguine about Iraq’s prospects without a U.S. presence.

Obama’s determination to gear down in Europe and the Middle East, regardless of circumstances, guaranteed that the United States would not respond strategically to new opportunities (the Arab Spring) or dangers (Putin’s determination to redraw the map of Europe).

When ordinary citizens in Tunisia, Egypt, Syria and elsewhere in the Arab world unexpectedly began agitating for democracy, the West might have responded as it did after World War II (with the Marshall Plan) or the fall of the Berlin Wall (with a commitment to a Europe whole and free). If the United States had taken the lead, Europe and America together could have offered trade, investment, exchange and cultural opportunities to help bring the region into the modern, democratic world.

But for Obama the tumult in Egypt and elsewhere was a distraction, not a once-in-a-generation opportunity. The West responded timidly and inconsistently, and the moment was lost.

For Russia, Obama offered Putin a “reset” strategy of improved relations. But when it became clear that Putin wasn’t interested — that he wanted to re-create a Russian empire while blocking the achievement of a Europe whole and free — the West again had no strategic response. Obama could have bolstered a unified Europe with military, diplomatic and trade measures. Instead, as Putin wrecked democracy in Russia, annexed Crimea and fomented war in Ukraine, Obama and his European counterparts were reactive and divided.

In Iraq and Syria, Obama’s predictions proved wrong. Without the 15,000 or so troops that U.S. generals hoped to station in Iraq for training and counterterrorism, the United States had no leverage as Iraq’s armed forces devolved into sectarian militias. When challenged by al-Qaeda, the army and the state itself quickly shattered.

Without Western backing, the moderate rebels in Syria are in retreat. Assad did not fall, and extremists — with a far more capable arsenal than the moderates have — established a state that Eric Holder finds “more frightening than anything I think I’ve seen as attorney general.”

Libya’s government, until recently spurned in its requests for help, gradually lost control. The country is now so dangerous that on Saturday the United States had to evacuate its embassy.

Syria did hand over the chemical weapons Assad acknowledged possessing, but the dictator was strengthened in the transaction. Even in Asia, the supposed pivot notwithstanding, allied leaders express doubts about U.S. commitment — and the reason they cite most often is Obama’s retreat from his red line in Syria.

To be sure, there are no true laboratory experiments in international relations. Even with different U.S. policies, the Arab Spring might have fizzled and the Iraqi army might have crumbled. No one can say for sure what would have happened if the United States had not signaled its exhaustion with foreign affairs, downgraded its interest in Europe and the Middle East, abandoned Iraq and stayed aloof from Syria.

But we can see what followed each of those strategic choices. Obama thought he could engineer a cautious, modulated retreat from U.S. leadership. What we have gotten is a far more dangerous world.